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Non Disjunction is a mistake that can be made during meiosis, when chromosomes or chromatids do not sort correctly. Spindle fibres pull both chromosomes/chromatids to one pole of the cell, rather than separating them to opposite ends. This affects the numbers of chromosomes in the gametes produced and thus will have an effect on the fertilised embryo. Non Disjunction can take place in both Meioses I and Meiosis II, and can take place at multiple positions (more than one pair of chromosomes/chromatids) within a cell.


Meiosis I

When Non-Disjunction takes place in Meiosis I, two homologous chromosomes are involved. Normally, one daughter cell would get one chromosome each, but in cases where Non-Disjunction is involved, both chromosomes will move to one pole of the cell. This causes one daughter cell and the gametes it splits into to have no chromosomes at that position. The other daughter cell will have 2 chromosomes at that position, and the gametes produced will get two chromatids each.

Meiosis II

If Meiosis I takes place as normal but Meiosis II has Non-Disjunction, the effect on the gametes will be a little different. Each cell produces by Meiosis I will have the normal amount of chromosomes (one at each position), and these will line up as normal to be sorted. However, Non-Disjunction will occur when the sister chromatids making up the chromosome do not separate correctly - thus one cell formed will get no chromatids and the other three. Therefore Non-Disjunction in Meiosis II usually causes the formation of two normal embryos(the other chromosome probably separated normally), one embryo with one chromatid and one with three. Therefore, after fertilisation and the addition of one chromosome at each position to the oocytes via a spermatocyte, two fertilised oocytes will have three chromosomes and two will have one.

Results of Non Disjunction

Non Disjunction causes Aneuploidy, where chromosome number is incorrect at a specific position. There are two potential types of Aneuploidy: Trisomy, where there are three chromosomes at a position, and Monosomy, where there is only a single chromosome at a position. Either of these can have severe effects.

Medical Associations

Trisomy at Position 21, known as Trisomy 21, causes Down Syndrome, which causes notable physical symptoms, mainly affecting appearance and causing learning difficulties including problems with memory and concentration, as well as delayed development[1].

Trisomy 18, known as Edwards Syndrome, causes mostly physical symptoms but have effects on the rate of mental development too[2].

Trisomy 13, known as Patau Syndrome, has severe mental effects and a range of physical symptoms, including missing skin and a decreased muscle tone[3].


  1. NHS (2010), Symptoms of Down's Syndrome, Available at: (30/11/2012
  2. U.S. National Library of Medicine (2012), Trisomy 18, Available at: (30/11/2012)
  3. U.S. National Library of Medicine (2011), Trisomy 13,, Available at: (30/11/2012)
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